Cohesive force refers to the attractive force between molecules of the same substance. In liquids, these forces are weaker than in solids but stronger than in gases.

Here's why:

* Liquid molecules are close together: The molecules in a liquid are in constant motion but are still relatively close together. This proximity allows for attractive forces to act between them.

* Intermolecular forces: These forces, such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces, are responsible for cohesion in liquids. The strength of these forces varies depending on the type of liquid.

* Surface tension: Cohesive forces are responsible for the phenomenon of surface tension, where the surface of a liquid acts like a stretched membrane. This is due to the inward pull of cohesive forces on the surface molecules.

Examples:

* Water molecules are highly cohesive due to strong hydrogen bonding.

* Mercury molecules have strong metallic bonding, leading to high cohesive forces.

* Oils have weaker cohesive forces compared to water, resulting in lower surface tension.

While all liquids have cohesive forces, their strength varies greatly. The specific properties of a liquid, like its viscosity and boiling point, are directly influenced by the strength of its cohesive forces.